Google's not indexing most pages in my sitemap file

I am perplexed. I created a sitemap file
(http://www.nauticalmind.com/sitemap.xml) (it's ~1.3MB) for the
bookstore I work at over a month ago. The file's verified etc., we're
collecting stats and some of our pages are being crawled by Google.

*But* many of our individual book pages --
e.g.
http://www.nauticalmind.com/customer/product.php?productid=66137&cat=3&page=6
-- are not being crawled. It seems that any book more than three links
away from our homepage (e.g. on the third page of some category
catalogue) aren't being looked at -- despite inclusion in the sitemap.

Any thoughts on what I'm doing wrong? Is it lack of patience or
something about the way our sitemap's setup? Any help is much
appreciated.